Scanning radiographic equipment differs from conventional radiography in that it employs a narrowly collimated beam of radiation, typically x-rays formed into, for example, a fan beam, rather than a broad area cone beam. The small beam size used in scanning radiographic equipment allows replacement of an image forming sheet of radiographic film, used with conventional radiographic equipment, with a small area array of detector elements.
The detector elements receiving the transmitted radiation produce electrical signals which may be converted to digital values by an analog to digital converter for the later development of an image or for other processing by computer equipment. The ability to quantify the measurement of the transmitted radiation, implicit in the digitization by the analog to digital converter, allows not only the formation of a radiographic "attenuation" image but also the mathematical analysis of the composition of the attenuating material by dual energy techniques. See generally, "Generalized Image Combinations in Dual KVP Digital Radiography", by Lehmann et al. Med. Phys. 8(5) September/October 1981. Such dual energy techniques quantitatively compare the attenuation of radiation at two energies to distinguish, for example, between bone and soft tissue. Dual energy techniques allow the measurement of bone mass, such measurement being important in the treatment of osteoporosis and other bone diseases.
The limited area of the beam of radiation used in scanning radiographic systems allows the use of limited area detectors permitting high resolution with relatively lower cost. The limited area of the detectors, however requires that the beam be scanned along several adjacent paths if large area images are to be constructed. Typically, a fan beam will be scanned in a raster pattern over the area to be measured, each line of the scan separated by somewhat less than the width of fan beam, to ensure complete illumination of the entire volume of the imaged object, with the directions of scanning being generally perpendicular to the direction of the radiation and the plane of the fan beam.
Images formed by a scanning radiographic system are potentially more accurate than those produced by a typical broad beam radiograph system. This accuracy arises from the limited divergence, in the scanning direction, of the rays of the fan beam, as compared to a broad area cone beam. This narrow collimation of the fan beam reduces "parallax" in the projected image, particularly of anatomical planar surfaces that are nearly parallel with the plane of the fan beam--such as the superior and inferior borders of the vertebrae in the spine when the scanning directions is along the superior-inferior axis of the body.
Morphological measurements of the vertebrae, and other structures, which benefit from reduced parallax are used to evaluate various dimensions of a vertebra to detect crushing or other deformation that are one element of certain bone diseases such as osteoporosis. See e.g. Minne et al., "A Newly Developed Spine Deformity Index (SDI) to Quantitate Vertebral Crush Factors in Patients with Osteoporosis," Bone and Mineral, 3:335-349 (1988); J. C. Gallagher et al, "Vertebral Morphometry: Normative Data," Bone and Mineral, 4:189-196 (1988); Hedlund et al, "Vertebral Morphometry in Diagnosis of Spinal Fractures," Bone and Mineral, 5:59-67 (1988); and Hedlund et al, "Change in Vertebral Shape in Spinal Osteoporosis," Calcified Tissue International, 44:168-172 (1989). Automatic techniques for morphological measurements of bone are described in U.S. patent application Ser. No. 07/944,626 filed Sep. 14, 1992 and entitled: "Method for Analyzing Vertebral Morphology Using Digital Radiography" assigned to the same assignee as the present application and hereby incorporated by reference.
Nevertheless, images developed with scanning fan beam equipment can include certain distortions or artifacts. In particular, it has been noted that objects at the interface between two adjacent scan paths contain a blurring or distortion in a direction perpendicular to the scan path.